Hiya,
If you don’t have enough pins on your micro, you might get away with setting up a ‘loop-back’ connection on the ws6318.
On the WS6318, connect CTS to RTS and DSR to DTR (and possibly CDC). Leave TxD and RxD connected to your micro.
Note that your micro will have to be able to handle the full data rate on the serial port, as the ws6318 will believe that it is always enabled to send data.
ciao, Dave